Purification of silver nitrate solutions



ilted States Patent r 2,940,828 PURIFICATION OF SILVER NITRATE SOLUTIONS Jerome Albert Moede, Rochester, N.Y., assiguor to E. 1. du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware 7 I I 1 No Drawing. Filed Oct. 29, 1957, Ser. No. 693,048

5 Claims. (Ci. 23-402) This invention relates to the purification of aqueous silver nitrate solutions. More particularly it relates to the removal of contaminating metal ions from aqueous solutions of silver nitrate.

Photographic silver halide emulsions are made by precipating silver halides in an aqueous colloid medium. The aqueous silver nitrate solutions used as the source of the silver ions should be very pure and free from other metal salts in order to obtain optimum photographic'properties. The prior art methods for the preparation of such solutions are quite time-consuming and tedious andrequire a plurality of crystallizations. i l

In U.S. Patent 2,543,792 which 'issued'Mar. 6, 1951, and U.S. Patent 2,614,029 which issued Oct. 14, 1952, processes of purifying aqueous silver nitrate solutions are described which are superior to the tedious and time-com suming methods of obtaining pure silver nitrate by repeated crystallization. However these processes while commercially practical have some disadvantages. 'Thus the process of U.S. Patent2,543,792 has the distadvantage that the alumina used must be replaced or regenerated frequently because it is exhausted rapidly bythe acid nature of the silver nitrate solution, and the relatively large quantities of metal impurities which must be removed. While the process of U.S. Patent 2,614,029 overcomes this disadvantage, both of the processes involve treatment of the solutions with activated carbon. This material is dusty and may introduce difficulties in handling because of its light powdery nature, 7 I

An object of this invention is to provide, an improved process for preparing silver nitrate solutions. having ,a high degree of purity. "Another object is to provide a simplified process for preparing silver nitrate solutions of good photographic quality. A further object is to provide a process which is free from the disadvantages of the processes described in the aforesaid patents. A still further object is to provide such a process which is economical and can be carried out on a relatively large scale. Still other objects will be apparent from the following description of the invention.

The above objects are accomplished and the disadvantages of prior art silver nitrate purification procedures overcome by the process of this invention which comprises; (a) treating an aqueous solution of silver nitrate which contains other metals and metal ions as impurities, e.g., copper, iron, lead, gold, bismuth, mercury, the platinum metals, etc., with sufiicient silver oxide to attain a pH of at least 6.1 and preferably between 6.1 and 9.0 (the pH attained with a given amount of silver oxide will vary with the concentration of the silver nitrate solution), (b) exposing the aqueous silver nitrate solution to ultraviolet light, (c) filtering the solution, and (d) passing it into contact with at least one water-insoluble, porous, solid basic adsorbent, e.g., activated alumina or magnesia.

In the event the solution recovered is still cloudy or hazy because of the presence of alumina or magnesia particles the solution can be passed through a microporous filter, e.g., a stainless steel microporous material or an inert microporous clay ceramic material or fritted glass.

Step :2 can be advantageously carried out by adding silver oxide to a 3 to 9 molar aqueous solution of silver nitrate which contains other heavy metal salts as impurities until the pH of the solution has been raised above 6 :1. The amount of silver oxide used will depend .onthe Patented June 14, 195.0

pH of the untreated solution which generally is between 0.5 and 3.6. The precipitate formed and any excess silver oxideare after ultraviolet treatment filtered off and the resulting silver nitrate solution removed and subjected to further treatment. lt has been determined that this step removes by precipitation as the, hydroxides or free metals practically all of the metal impurities which behave as poisons in photographic emulsions andwhich are normally present in nitric acid and silver bullion;

The treatment of the material with ultravioletlight following step (a) removes ions ofmetalsjof the platinuni group, gold, bismuth and mercury and photographically harmful organic impurities remaining in the supernatant solution after the treatment with silver oxide. The aqueous silver nitrate solution which is recovered after the ultraviolet light treatment ,can then be brought into intimate contact with particles or pieces of activated alumina or magnesia preferably ,by passing it through a column packed with same. In general, there shouldbe from 2 to 10% by weightfof activated alumina based on the amount of silver nitrate in theoriginal solution.

The activated alumina or'magnesia are also preferably in fine granular form having an average particle size of 28-400 mesh. It has beendetermined that this step removes substantially all of the metal ion impurities which remain in the solution. Treatment of the alumina to coat it with silver oxide and the use of silver oxide-coated glass .filters as shown in U.S. Patent 2,614,029 can also The aqueous silver nitrate solution recoveredgfrom the activated alumina may sometimes contain fin Particles of alumina in suspension and they can berernovedby any practical method of removing fine particles :from solution so that they will not'cause spots in thefinijshed films'or papers. The removal of theparticleszcanbe advantageously accomplished by filteringthesolution through a Stainless steel filter in i rage-:poresize of 5' microns or less.

. After any, alumina or magnesia particles have been 're moved, the pH ofthe silver nitrate solution can bev adjustedto .-3.0-3,.;5 with dilute nitric acid and the solution is then of such high degree of1purityrthatit can be used directly for the precipitation of light-sensitive silver halides in aphotographiecolloid emulsion. l

In carrying out the'processl of the present invention OH iderable flexibility. of operationis possible. The-silver used can lee-relatively impure including commercial silve bullion in bar, pellet or crystal formand thenitric acids can be QR, technical or commercial grade. The silver oxide used inthe process can be the dry'pulverized form or used in the form of a freshly prepared paste or slurry as describedin Example V of. U.S. 12,614,029; the latter formbeing preferred. 7 I I The ultraviolet light treatment can be carried out using a -Wld6 variety of equipmentand-light sources. Suitable light sources includecarbon arcs, mercury vapor arcs, fluorescent lamps with special "ultraviolet light emitting phosphors, argon glow lamps and photographic flood lamps. Of these the mercury vapor arcs, e.g., commercial" type sun-lamps are preferred. The ultraviolet lamps can be used singly or in groups and the equipment arranged in any convenient manner to irradiate the solutions being" treated. The time of irradiation can be adjusted over a wide range from 15 minutes .or less ,to 5 hours or more depending upon the purity of the silver nitrate to be I treated, the concentration of the solution and the depth sels is shown in the examples it will be understood within providing a high "proportion-bf illumination; Freshial'uininu 'used'when theads rptive. capacitybegins to'dimim but regeneration of the adsorbents is e onem eali and has intended to be limitedbyithefollovving 'eX' mples:

the scope of this invention that silver nitrate solutions l can be treated continuously in thin films flowing past a V bank of ultraviolet radiation emitting lamps or in transparent pipes-zot quartz or ultraviolet radiation transmitti'rig'glass orQpIasticsQ ,Ultraviolet radiation. having a wavelength within therange lfof'about l850f'to about 3800ang' stroms 'is's iitab 'violetiradiatin can, b

length 2557a.aieesa ei lyngermj; V

" agne'sium Oxide'can be other. advantages 'Alumiiiajcanfbe re'generated iiithe ad- "sorption columnfby washing fii'sfwithconcentratdnitric acid; e.g'.,"33-70%; follovved by dilute nitric acid, e.g., 0.5 to 1% andthenbyldistilled' water, afterfwhichTthe V columnjis washedwith'sodium hydroxide (about 0.25 to 1 ,3; molar. and againivv'ith distilled waterl 'It' is preferable that th d and seused. in washing'the column beiof C P. gr The q entities and rates" of flowv can the pH,co nductivityand/or c cal content. The invention will be furtlie illustrated.but it"i {A solution-pf silver nitrate lvvas prepared fr'om' 9919 silverbullion by dissolving it in59%; commercial grade "nitric 'acid using-100 parts byfvveightbullion toaboiit 152parts'by Weight of nitric acid; The acid Was jadded j in portibnstoth silver bullion to'maintain the tempera V ture in .therange of 71? to 82 C. The temperaturein l the reaction vessel 'vvasmaintainedconstant for about minutes; and .then raised-to the-boiling point and boiled untiltheresultingj'solution attained a'pH of about 0.5; Uudissolved silve'r wa's then filtered ofl? and "the solu} solution was diluted vvitli-distilled water'to 6N;

r Twenty-fourtotwenty-five gallons of the silvernitrate solution fdescribed above' in a '35 gallon kettle'fur'nis hfed with a stirrer was treated asdescribed in UfSf'Patefif; f 2614;029?by the'addition' ofsi'lverjoii i'de'; until the V so 1 3 t6,

increased to a 'value ab ove' 6-.1. -Stirring"time i and'athe solution irradiated by radiat-ion from a General Pt Hg Ir Ru" 'Rh Na less less less 1 f less less j than fthan W than than than; 'fthan' j' r The silver nitrate prepared by the above procedure was usd to prepare a positivetype gelatinoi's-ilver iodobro- 'mide emulsion having '1 mole percent iodide and the rest bromide and compared to a control emulsion prepared by the same procedure using a. GP. gradefof silver nitrate .commOn y l sed in the manufacture of photographic emulsions and manufactured by the repeated crystallization. g i

The films tested were exposedjn a sensitometerand developed at, 68 F.ffor ;3 .5{rn inutes in'a developer consi t Qt -4 m. N-me h lm ph ne l a e 3- tion boiled 'for another "sixty iminutes afterlWhi the J Electric RS sunlainp placed about; one foot? frog-met surface 'of l the-i'solution for a period ot 2 hours The V a solution ,depth in the kettle was about 3 fe'etI- The Geri-- eral Electric R'sgsunlamp is described in General Electric Company Lamp BulletinLD-l as a mercury 'vapor lamp having a 100 wattme rcury discharge e'lement and'a"1 watt tungsten filamentb'allast in a frosted glass envelope. This lamp is operated; on '60 cycle 110 volt alternating d pumped through onepart' Theisolution was filtered overa pad of glass-wool in .a'colunm having'a ratio of height towidth of 4 to 1, The solution was then filtered into aistainless.steeltreceivingkettle. After purificationthe pH jwas adjusted. to 3.0'with l N nitric acid and 7 crystals of silver nitrate of highpurity were prepared from the solution by evaporation and crystallization."

Data from the spectrographic analysis of the silver nitratercrystals obtained by the above process are summarized below; r

'Metdl impurity grams be! 10 grams silver nitrate Pd Au on Pb Bil I less less less less less than than than than an 0.05 0.25, 5.0 1.5 0.0025

current and requires 3 -minutes'to reach full ultraviolet 3; 1 output of about 35,000 E-vitons,

' of activated alumina, 28-48 mesh; for'15.8 parts at solid silver nitrate in the solution, the alumina being packed an emulsion' m'adejin jan ;identical "n anriir are, shown sm -i y e qne =2 si -I mv u fi l01-. m of the mono-hydrate of -sodium'-carbonate; and 0;44 gm. of; potassium b romide, 7 all: diluted up to --one liter, and fixed, washed and dried in aconventional manner. 7

The speed, and D 'gammaandf og values for the two '5 n i l n 'fw r s ss ls i ;t9 ehsthi as w Test Em u ion lcputrol Einulsib n ts at F; at

60%RHTime I 7 r I *(month); Rel. Hand'D Fog 'Rel.-- =11 andD -Fog Speed. Gamma Speed "Gamma 7 of ,ov the nitrate high speed negative type gelatino silver'iodobromide emulsion having 3.4% iodide compared with C;P,.. grade silver nitrate in p am p en re was prepared 2.65 liters of 858 N' silver'nitrate- .solutionusin'glfiP. g rade nitric acid and a"mixture of silvei'fibars and crystalsiranging iri' purity from 99.9% to:

99.97%? :'A 1425:1111: portion of thisusolution Was .ad justed'toapI-Iv'a bove 6";0iby sl'owly' adding freshly pr'e-r cipitated and washed 'silvenoxide .lvvithconstantstirring The mixture was diluted Wlihi'ZSO ml; of waterandtheni exposed fo'r90 mimitestotheradiation from aHanovia quartz mercury .vapor lampl which,-;wl1en' operated. on 60 cycle 115. volt cnr'rentytdraws.iapproxirnately ;watts andprovides an 'illuminati on at; 20 inches of 1aboYe250 microwatts'imeasured. at wavelengths of 3130' A; and lower; "The lamp was positiofied :close fothe surface of. the solution in a two liter beaker andthe solution stirred during-exposure; "The solutionwas thenfiltere'd through glas's' vvool and a mediump'orosityr glass 1discfand thefilter washed with smallaportions of distilled water; All

'- but;250 m1. ofthe solution which Was removed for analyticalpurposes was passed through a" column contain.

conventional method of ing 75 grams of aluminum oxide adsorbent and; the pH adjusted to 2.9. Data from the spectrographic analysis of the silver nitrate solution obtained by the above process is summarized below:

Metal impurity grams per 10 grams silver nitrate Pd Au Cu Pb Bi less less less less less than than than than than 0. 15 0. 5 5. l. 0.01

Pt Hg Ir Ru Rh Na less less less less less less than than than than than than 1. 0 0. 3 02 03 0. 04 60 The silver nitrate prepared by the above procedure was used to prepare an X-ray type gelatino silver iodobromide photographic emulsion having 1.55 mole percent iodide and the rest bromide, and compared with a control emulsion prepared by the same procedure using a Cl. grade of silver nitrate commonly used in the manufacture of photographic emulsion and manufactured by the conventional method of repeated crystallization. The films tested were developed at 68 F. for five minutes in a developer consisting of 5.0 gms. N-methyl-p-aminophenol sulfate, 7.5 gms. hydroquinone, 60.0 gms. sodium sulfite, 58.5 gms. of the mono-hydrate of sodium carbonate, and 4.5 gms. of potassium bromide, all diluted up to one liter, and fixed, washed and dried in a conventional manner:

This invention has the advantage that it provides a practical process for purifying aqueous silver nitrate solutions on a commercial scale. A further advantage resides in the fact that the first step alone of adding sufi'icient silver oxide to attain a pH of at least 6.1 removes more than 90% of the metal impurities present in silver nitrate obtained from silver bullion and commercial nitric acid. The use of silved oxide before treatment with alumina has the advantage that the alumina does not have to be replenished or regenerated as frequently and a smaller amount of this material can be used. The ultraviolet radiation exposure is used in place of the carbon black treatment of US. 2,614,029. The ultraviolet treatment is advantageous because it eliminates the dust, difiiculties in handling because of its light powdery nature, and cost of the carbon adsorbent. Yet another advantage resides in the fact that more uniform purified silver nitrate solutions can be made from difierent lots of solutions treated.

As many widely difierent embodiments of this invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not to be limited except as defined by the claims.

What is claimed is:

1. A process of purifying an aqueous solution of silver nitrate containing small amounts of heavy metal and noble metal impurities which consists of treating said solution with silver oxide to attain a pH of at least 6.1, exposing said solution to ultraviolet radiation having a. wavelength within the range of about 1850 to about 3800 angstroms in an amount of from about to about 1750 microwatt hours of ultraviolet radiation per gram of silver nitrate treated, separating the silver nitrate solution from the precipitate, and passing the resulting solution into contact with at least one water-insoluble porous solid adsorbent taken from the group consisting of alumina and magnesia,

2. A process of purifying an aqueous solution of silver nitrate containing small amounts of heavy metal and noble metal impurities which consists of treating said solution with silver oxide to attain a pH ofat least 6.1, treating said solution with ultraviolet radiation having a Wavelength within the range of about 1850 to about 3800 angstroms in an amount of from about 100 to about 1750 microwatt hours of ultraviolet radiation per gram of silver nitrate treated, separating the silver nitrate solution from the precipitated metals and metal hydroxides and silver oxide, and passing the resulting solution into contact with alumina.

3. A process of purifying an aqueous solution of silver nitrate containing small amounts of heavy metal and noble metal impurities which consists of treating said solution with silver oxide to attain a pH between 6.1 and 9.0, exposing said solution to ultraviolet radiation having a wavelength within the range of about 1850 to about 3800 angstroms in an amount of from about 100 to about 1750 microwatt hours of ultraviolet radiation per gram of silver nitrate treated, allowing the precipitated metals, metal hydroxides and silver oxide to separate out, removing the silver nitrate solution, and bringing the resulting solution into intimate contact with particles of adsorbent material taken from the group consisting of alumina and magnesia.

4. A process for purifying an aqueous solution of silver nitrate containing small amounts of heavy metal and noble metal impurities which consists of treating said solution with silver oxide to attain a pH of 6.1 to 9.0, exposing said solution to ultraviolet radiation having a wavelength within the range of about 1850 to about 3800 angstroms in an amount of from about 100 to about 1750 microwatt hours of ultraviolet radiation per gram of silver nitrate treated, allowing the precipitated metals, metal hydroxides and any silver oxide to separate out, removing the silver nitrate solution, bringing said solution into contact with activated alumina, and additionally filtering said solution.

5. A process of purifying an aqueous solution of silver nitrate containing small amounts of heavy metal and noble metal impurities which consists of treating said solution with silver oxide to obtain a pH of from 6.1 to 9.0, exposing said solution to ultraviolet radiation having a high proportion of illumination at about 2537 A. wavelength in an amount of from about 100 to about 1750 microwatt hours of ultraviolet radiation per gram of silver nitrate treated, allowing the precipitated metals, metal hydroxides and any silver oxide to separate out, removing the silver nitrate solution, and bringing the resulting solution into contact with from 2 to 10% by weight based on the silver nitrate of finely divided activated alumina having an average particle size of 28-100 mesh.

References Cited in the file of this patent UNITED STATES PATENTS 873,508 Cowper-Bowles Dec. 10, 1907 2,207,566 Waldeck July 9, 1940 2,282,294 Coey May 5, 1942 2,614,029 Moede Oct. 14, 1952 FOREIGN PATENTS 17,790 Great Britain July 20, 1914 of 1913 OTHER REFERENCES Hackhs Chemical Dictionary, 3rd ed., 1944, Blakiston, pp. 878-79. 

1. A PROCESS OF PURIFYING AN AQUEOUS SOLUTION OF SILVER NITRATE CONTAINING SMALL AMOUNTS OF HEAVY METAL AND NOBLE METAL IMPURITIES WHICH CONSISTS OF TREATING SAID SOLUTION WITH SILVER OXIDE TO ATTAIN A PH OF AT LEAST 6.1, EXPOSING SAID SOLUTION TO ULTRAVIOLET RADIATION HAVING A WAVELENGTH WITHIN THE RANGE OF ABOUT 1850 TO ABOUT 3800 ANGSTROMS IN AN AMOUNT OF FROM ABOUT 100 TO ABOUT 1750 MICEOWATT HOURS OF ULTRAVIOLET RADIATION PER GRAM OF SILVER NITRATE TREATED, SEPARATING THE SILVER NITRATE SOLUTION FROM THE PRECIPITATE, AND PASSING THE RESULTING SOLUTION INTO CONTACT WITH AT LEAST ONE WATER-INSOLUBLE POROUS SOLID ADSORBENT TAKEN FROM THE GROUP CONSISTING OF ALUMINA AND MAGNESIA. 